1. Field of the Invention
The present invention relates to the photopolymerization of hydrophilic monomers and to the water soluble polymers resulting therefrom, as well as the use of such polymers as polymeric flocculants.
2. Description of the Prior Art
It is well known to this art to prepare polymeric flocculating agents via photopolymerization of acrylic monomers, specifically acrylamide, methacrylamide, acrylic or methacrylic acids, optionally the salts thereof, and quaternary ammonium salts derived from dialkylaminoalkyl (meth)acylates.
A plurality of characteristics and properties is typically required of such organic polymeric flocculating agents and of the respective processes for the preparation thereof, certain of which are often contradictory and mutually inconsistant, i.e.,: the polymeric flocculating agents must be water soluble; they must have molecular weights as high as possible (high intrinsic viscosity); when dissolved in water, they must, if possible, leave no insoluble residue; their speed of dissolution in water must be as high as possible; it must be possible to produce such polymeric flocculants by polymerization of an aqueous monomer solution which is as highly concentrated as possible in order that the output of the production facility will be high, while, simultaneously, the heat generated during polymerization, or exotherm, must be quickly dissipated such that as low a temperature as possible is maintained in the polymerization recipe to prevent degradation of polymer, a reduction in molecular weight, and the possible formation of insoluble fractions in the water; and, finally, it is necessary that the polymerization quickly proceed to as advanced a stage as is possible, in order that the content of residual monomer or monomers in the flocculant be as low as possible.
It is moreover, quite difficult to attain and reconcile all of the foregoing objectives when the monomers are pure, but it is even more difficult when the monomers contain impurities.
In our aforenoted copending application, there is disclosed and claimed a certain improved process for the thin layer photopolymerization of olefinically unsaturated hydrophilic monomers into high quality, water soluble polymeric flocculants, even in the event that the starting material monomers are not the highest in purity. Such process features the continuous deposition, onto a traveling endless conveyor belt, of a thin layer or film of an aqueous solution of the monomer(s), said solution comprising a photopolymerization promoter and a particular solubilization facilitating polyhydroxy additive, and then exposing the deposited thin layer to irradiation with light rays having a wavelength between 150 and 500 m.mu..
In the basic thin layer photopolymerization process of the type disclosed in our said copending application, it is a characteristic thereof that the viscosity of the medium undergoing photopolymerization progressively increases, i.e., the polymerizing liquid becomes more and more viscous, is transformed into a gel, and then into a layer of rubbery consistency, and/or, in certain instances, into a hard solid layer.
As above indicated, the production of water soluble polymeric flocculating agents essentially devoid of a fraction of insolubles is a desideratum in this art. Unfortunately, however, most of the photopolymerizations designed to afford same are characterized by reaction parameters which favor the by-production of just such insoluble fractions. Among such parameters, there are mentioned:
[i] Increasing the duration and/or the intensity of irradiation for purposes of reducing the content of residual monomer in the resultant flocculating agent;
[ii] Acceleration of the photopolymerization by any one of a great variety of means;
[iii] Increasing the monomer(s) concentration of the aqueous solutions subjected to irradiation;
[iv] Utilizing operating conditions which favor production of high molecular weight polymers (i.e., polymers having high intrinsic viscosities);
[v] The use of UV radiation lamps of high thermal energy (particularly high pressure mercury lamps); and
[vi] Lowering the pH of the photopolymerization medium, especially the accidental lowering of pH.
Moreover, it further appears that the insoluble fractions, if any be present, are not all of the same appearance and/or nature. Some are flocculent or gelatinous in appearance. Others are foliated, often inwardly curving or reflecting an "onionskin" design.
Finally, even if a given photopolymerization be devised for the bench scale or pilot plant production of completely water soluble polymers, when adapting same for industrial applications it often happens that slight process parameter variations or modifications [the so-called "accidental" modifications] at one time or another do in fact give rise to the by-production of objectionable insoluble fractions, or fractions which are more difficultly soluble in water.
Thus, a serious need continues to exist for photopolymerization techniques, other than that described in our said copending application, designed for the preparation of the wholly water soluble polymeric flocculants.